Continuation. See the beginning in № 15, 16/2004

Lesson 5. Hybridization
atomic orbitals of carbon

A covalent chemical bond is formed using common bonding electron pairs of the type:

Form a chemical bond, i.e. only unpaired electrons can create a common electron pair with an "alien" electron from another atom. When writing electronic formulas, unpaired electrons are located one at a time in the orbital cell.
Atomic orbital Is a function that describes the density of an electron cloud at each point in space around the nucleus of an atom. An electron cloud is a region of space in which an electron can be found with a high probability.
To reconcile the electronic structure of the carbon atom and the valence of this element, the concept of the excitation of a carbon atom is used. In the normal (unexcited) state, a carbon atom has two unpaired 2 R 2 -electron. In an excited state (when energy is absorbed) one of 2 s 2 -electrons can go to the free R-orbital. Then four unpaired electrons appear in the carbon atom:

Recall that in electronic formula atom (for example, for carbon 6 С - 1 s 2 2s 2 2p 2) large numbers in front of the letters - 1, 2 - indicate the number of the energy level. Letters s and R indicate the shape of the electron cloud (orbital), and the numbers to the right above the letters indicate the number of electrons in that orbital. Everything s- spherical orbitals:

On the second energy level, except for 2 s-orbitals there are three 2 R-orbital. These 2 R-orbitals have an ellipsoidal shape, similar to dumbbells, and are oriented in space at an angle of 90 ° to each other. 2 R-Orbitals stand for 2 p x, 2p y and 2 p z according to the axes along which these orbitals are located.

When chemical bonds are formed, the electron orbitals acquire the same shape. So, in saturated hydrocarbons one mixes s-orbital and three R-orbitals of a carbon atom with the formation of four identical (hybrid) sp 3-orbitals:

It - sp 3-hybridization.
Hybridization- alignment (mixing) of atomic orbitals ( s and R) with the formation of new atomic orbitals, called hybrid orbitals.

Hybrid orbitals have an asymmetric shape, elongated towards the attached atom. The electron clouds repel each other and are located in space as far from each other as possible. In this case, the axes of four sp 3-hybrid orbitals turn out to be directed to the vertices of the tetrahedron (regular triangular pyramid).
Accordingly, the angles between these orbitals are tetrahedral, equal to 109 ° 28 ".
The vertices of the electron orbitals can overlap with the orbitals of other atoms. If the electron clouds overlap along the line connecting the centers of the atoms, then such a covalent bond is called sigma () - communication... For example, in a C 2 H 6 ethane molecule, a chemical bond is formed between two carbon atoms by overlapping two hybrid orbitals. It's a connection. In addition, each of the carbon atoms has its own three sp 3-orbitals overlap with s-orbitals of three hydrogen atoms, forming three -bonds.

In total, three valence states with different types of hybridization are possible for a carbon atom. except sp 3-hybridization exists sp 2 - and sp-hybridization.
sp 2 -Hybridization- mixing one s- and two R-orbitals. As a result, three hybrid sp 2 -orbitals. These sp 2 -orbitals are located in the same plane (with the axes NS, at) and directed to the vertices of the triangle with an angle between the orbitals of 120 °. Unhybridized
R-orbital perpendicular to the plane of the three hybrid sp 2 -orbitals (oriented along the axis z). Upper half R-orbital is above the plane, the lower half is below the plane.
Type of sp 2-hybridization of carbon occurs in compounds with a double bond: C = C, C = O, C = N. Moreover, only one of the bonds between two atoms (for example, C = C) can be a bond. (The other bonding orbitals of the atom are directed in opposite directions.) The second bond is formed by overlapping non-hybrid R-orbitals on either side of the line connecting the atomic nuclei.

Covalent bond formed by lateral overlap R-orbitals of adjacent carbon atoms is called pi () - communication.

Education -connection

Because of the less overlapping of the orbitals, the β-bond is less strong than the β-bond.
sp-Hybridization Is mixing (alignment in shape and energy) of one s- and one
R-orbitals with the formation of two hybrid sp-orbitals. sp-Orbitals are located on the same line (at an angle of 180 °) and are directed in opposite directions from the nucleus of the carbon atom. Two
R-orbitals remain unhybridized. They are placed mutually perpendicular
directions of communication. On the image sp-orbitals are shown along the axis y, and unhybridized two
R-orbitals - along the axes NS and z.

The CC triple carbon-carbon bond is composed of a β-bond arising from overlapping
sp-hybrid orbitals, and two -bonds.
The relationship between such parameters of the carbon atom as the number of attached groups, the type of hybridization and the types of chemical bonds formed is shown in Table 4.

Table 4

Covalent carbon bonds

Number of groups related with carbon	Type of hybridization	Types participating chemical bonds	Examples of compound formulas
4	sp 3	Four - ties
3	sp 2	Three - ties and one - bond
2	sp	Two - ties and two -connections	H – CC – H

Exercises.

1. What electrons of atoms (for example, carbon or nitrogen) are called unpaired?

2. What does the concept of "common electron pairs" mean in compounds with a covalent bond (for example, CH 4 or H 2 S )?

3. What are the electronic states of atoms (for example, C or N ) are called basic, and which are excited?

4. What do the numbers and letters mean in the electronic formula of the atom (for example, C or N )?

5. What is an atomic orbital? How many orbitals on the second energy level of atom C and how do they differ?

6. What is the difference between hybrid orbitals and the original orbitals from which they were formed?

7. What types of hybridization are known for a carbon atom and what are they?

8. Draw a picture of the spatial arrangement of the orbitals for one of the electronic states of the carbon atom.

9. What chemical bonds are called and what? Please indicate-and-connections in connections:

10. For the carbon atoms of the compounds below, indicate: a) the type of hybridization; b) the types of its chemical bonds; c) bond angles.

Answers to exercises for topic 1

Lesson 5

1. The electrons that are one at a time in the orbital are called unpaired electrons... For example, in the electron diffraction formula of an excited carbon atom, there are four unpaired electrons, while a nitrogen atom has three:

2. Two electrons participating in the formation of one chemical bond are called common electronic pair... Usually, before the formation of a chemical bond, one of the electrons of this pair belonged to one atom, and the other electron belonged to another atom:

3. The electronic state of an atom, in which the order of filling of electron orbitals is observed: 1 s 2 , 2s 2 , 2p 2 , 3s 2 , 3p 2 , 4s 2 , 3d 2 , 4p 2, etc., are called ground state... V excited state one of the valence electrons of the atom occupies a free orbital with a higher energy, such a transition is accompanied by the separation of paired electrons. Schematically, it is written as follows:

Whereas in the ground state there were only two valence unpaired electrons, in the excited state there are four such electrons.

5. An atomic orbital is a function that describes the density of an electron cloud at each point in space around the nucleus of a given atom. At the second energy level of the carbon atom, there are four orbitals - 2 s, 2p x, 2p y, 2p z... These orbitals are different:
a) the shape of the electron cloud ( s- ball, R- dumbbell);
b) R-orbitals have different orientations in space - along mutually perpendicular axes x, y and z, they are denoted p x, p y, p z.

6. Hybrid orbitals differ from the original (non-hybrid) orbitals in shape and energy. For example, s-orbital - the shape of a sphere, R- symmetrical figure eight, sp-hybrid orbital - asymmetric figure eight.
Energy Differences: E(s) < E(sp) < E(R). Thus, sp-orbital - the orbital averaged over shape and energy, obtained by mixing the original s- and p-orbitals.

7. Three types of hybridization are known for a carbon atom: sp 3 , sp 2 and sp (see lesson 5 text).

9. -bond - a covalent bond formed by head-on overlapping of orbitals along a line connecting the centers of atoms.
-bond - a covalent bond formed by lateral overlapping R-orbitals on either side of the line connecting the centers of the atoms.
-Bond is shown by the second and third dash between the connected atoms.

By hybridizationis called a hypothetical mixing process of various types, but close in energy, orbitals of a given atom with the appearance of the same number of new (hybrid 1) orbitals, identical in energy and shape.

Hybridization of atomic orbitals occurs during the formation of covalent bonds.

Hybrid orbitals have the shape of a three-dimensional asymmetric figure eight, strongly elongated to one side of the atomic nucleus:.

This shape causes a stronger overlap of hybrid orbitals with orbitals (pure or hybrid) of other atoms than in the case of pure atomic orbitals and leads to the formation of stronger covalent bonds. Therefore, the energy spent on the hybridization of atomic orbitals is more than compensated for by the release of energy due to the formation of stronger covalent bonds with the participation of hybrid orbitals. The name of the hybrid orbitals and the type of hybridization are determined by the number and type of atomic orbitals involved in hybridization, for example: sp-, sp 2 -, sp 3 -, sp 2 d- orsp 3 d 2 -hybridization.

The directionality of the hybrid orbitals, and hence the geometry of the molecule, depend on the type of hybridization. In practice, the inverse problem is usually solved: first, the geometry of the molecule is established experimentally, after which the type and shape of the hybrid orbitals involved in its formation are described.

sp -Hybridization. Two hybrid sp- as a result of mutual repulsion, the orbitals are located relative to the atomic nucleus in such a way that the angle between them is 180 ° (Fig. 7).

Rice. 7. Mutual arrangement in the space of two sp- hybrid orbitals of one atom: a - surfaces covering areas of space where the probability of an electron being present is 90%; b - conditional image.

As a result of this arrangement of hybrid orbitals, molecules of the composition AX 2, where A is the central atom, have linear structure, that is, the covalent bonds of all three atoms are located on one straight line. For example, in a state sp- hybridization are the valence orbitals of the beryllium atom in the BeCl 2 molecule (Fig. 8). Linear configuration due to sp- Hybridizations of valence orbitals of atoms are also observed in BeH 2, Be (CH 3) 2, ZnCl 2, CO 2, HC≡N and a number of others.

Rice. 8. Three-atom linear molecule of beryllium chloride BeCl 2 (in a gaseous state): 1 - 3R- Cl atom orbital; 2 - two sp- hybrid orbitals of the Be atom.

s R 2 -Hybridization. Consider the hybridization of one s- and two R- orbitals. In this case, as a result of a linear combination of three orbitals, three hybrid sR 2 -orbital. They are located in the same plane at an angle of 120 ° to each other (Fig. 9). sR 2 -Hybridization is characteristic of many boron compounds, which, as shown above, in an excited state has three unpaired electrons: one s- and two R-electron. Overlapping sR 2 -orbitals of the boron atom with the orbitals of other atoms, three covalent bonds are formed, equivalent in length and energy. Molecules in which the valence orbitals of the central atom are in the state sR 2 -hybridization, have a triangular configuration. The angles between the covalent bonds are 120 °. Capable of sR 2 -hybridization are the valence orbitals of boron atoms in BF 3, BC1 3 molecules, carbon and nitrogen atoms in CO 3 2 -, NO 3 - anions.

Rice. 9. Mutual arrangement in space of three sR 2 -hybrid orbitals.

s R 3 -Hybridization. Substances are very widespread in the molecules of which the central atom contains four sR 3 -orbitals formed as a result of a linear combination of one s- and three R-orbitals. These orbitals are located at an angle of 109˚28 ′ to each other and directed to the vertices of the tetrahedron, in the center of which there is an atomic nucleus (Fig. 10 a).

Formation of four equivalent covalent bonds due to overlapping sR 3 -orbitals with orbitals of other atoms is characteristic of carbon atoms and other elements of the IVA group; this determines the tetrahedral structure of molecules (CH 4, CC1 4, SiH 4, SiF 4, GeH 4, GeBr 4, etc.).

Rice. 10. Influence of non-bonding electron pairs on the geometry of molecules:

a- methane (no non-binding electron pairs);

b- ammonia (one non-binding electron pair);

v- water (two non-binding pairs).

Lonely electron pairs hybrid orbit lei . In all the examples considered, the hybrid orbitals were "populated" by single electrons. However, it is not uncommon for a hybrid orbital to be "populated" by an electron pair. This affects the geometry of the molecules. Since the non-bonding electron pair is influenced by the nucleus of only its own atom, and the bonding electron pair is under the influence of two atomic nuclei, the non-bonding electron pair is closer to the atomic nucleus than the bonding one. As a result, the non-bonding electron pair repels the bonding electron pairs more strongly than they repel each other. Graphically, for clarity, the large repulsive force acting between the non-bonding and bonding electron pairs can be depicted as a larger electron orbital of the non-bonding pair. A non-binding electron pair exists, for example, at the nitrogen atom in the ammonia molecule (Fig. 10 b). As a result of interaction with bonding electron pairs, the H-N-H bond angles are reduced to 107.78 ° compared to 109.5 °, characteristic of a regular tetrahedron.

An even greater repulsion is experienced by bonding electron pairs in a water molecule, where the oxygen atom has two non-bonding electron pairs. As a result, the valence angle H-O-H in a water molecule is 104.5 ° (Fig. 10 v).

If a non-bonding electron pair, as a result of the formation of a covalent bond by the donor-acceptor mechanism, turns into a bonding one, then the repulsive forces between this bond and other covalent bonds in the molecule equalize; the angles between these bonds are also aligned. This happens, for example, with the formation of an ammonium cation:

Participation in hybridization d -orbitals. If the energy of atomic d- orbitals is not very different from energies s- and R- orbitals, then they can participate in hybridization. The most common type of hybridization involving d- orbitals is sR 3 d 2 - hybridization, as a result of which six hybrid orbitals of equivalent shape and energy are formed (Fig. 11 a), located at an angle of 90˚ to each other and directed to the vertices of the octahedron, in the center of which there is an atomic nucleus. Octahedron (Fig. 11 b) – is a regular octahedron: all edges in it equal length, all faces are regular triangles.

Rice. eleven. sR 3 d 2 - Hybridization

Less common sR 3 d- hybridization with the formation of five hybrid orbitals (Fig. 12 a) directed to the vertices of the trigonal bipyramid (Fig. 12 b). A trigonal bipyramid is formed by joining two isosceles pyramids with a common base - a regular triangle. Bold strokes in Fig. 12 b edges of equal length are shown. Geometrically and energetically sR 3 d- hybrid orbitals are unequal: three "equatorial" orbitals are directed to the vertices of a regular triangle, and two "axial" - up and down perpendicular to the plane of this triangle (Fig. 12 v). The angles between the "equatorial" orbitals are equal to 120 °, as for sR 2 - hybridization. The angle between the "axial" and any of the "equatorial" orbitals is 90 °. Accordingly, the covalent bonds that are formed with the participation of "equatorial" orbitals differ in length and energy from the bonds in the formation of which the "axial" orbitals are involved. For example, in the PC1 5 molecule, the “axial” bonds are 214 pm long, and the “equatorial” ones are 202 pm long.

Rice. 12. sR 3 d- Hybridization

Thus, considering covalent bonds as a result of overlapping atomic orbitals, one can explain the geometry of the resulting molecules and ions, which depends on the number and type of atomic orbitals involved in the formation of bonds. The concept of hybridization of atomic orbitals, it is necessary to understand that hybridization is a conditional technique that allows you to visually explain the geometry of a molecule through a combination of AOs.

Hybridization concept

The concept of hybridization of valence atomic orbitals was proposed by the American chemist Linus Pauling to answer the question why, when the central atom has different (s, p, d) valence orbitals, the bonds formed by it in polyatomic molecules with the same ligands are equivalent in their energy and spatial characteristics.

The concept of hybridization is central to the valence bond method. Hybridization itself is not a real physical process, but only a convenient model to explain electronic structure molecules, in particular, hypothetical modifications of atomic orbitals during the formation of a covalent chemical bond, in particular, the alignment of the lengths of chemical bonds and bond angles in a molecule.

The concept of hybridization was successfully applied to the qualitative description of simple molecules, but was later extended to more complex ones. Unlike the theory of molecular orbitals, it is not strictly quantitative, for example, it is unable to predict the photoelectron spectra of even such simple molecules as water. It is currently used mainly for methodological purposes and in synthetic organic chemistry.

This principle is reflected in the theory of repulsion of electron pairs Gillespie - Nyholm. First and most important rule which was formulated as follows:

"The electron pairs take such an arrangement on the valence shell of the atom, in which they are maximally distant from each other, that is, the electron pairs behave as if they were mutually repelling."

The second rule is that "All electron pairs included in the valence electron shell are considered to be located at the same distance from the nucleus".

Hybridization types

sp hybridization

Occurs when one s- and one p-orbitals are mixed. Two equivalent sp-atomic orbitals are formed, located linearly at an angle of 180 degrees and directed in different directions from the nucleus of the carbon atom. The two remaining non-hybrid p-orbitals are located in mutually perpendicular planes and participate in the formation of π-bonds, or engage in lone electron pairs.

sp 2 -hybridization

Occurs when one s and two p orbitals are mixed. Three hybrid orbitals are formed with axes located in the same plane and directed to the vertices of the triangle at an angle of 120 degrees. Non-hybrid p-atomic orbital perpendicular to the plane and, as a rule, participates in the formation of π-bonds

sp 3 -hybridization

It occurs when one s- and three p-orbitals are mixed, forming four sp3-hybrid orbitals of equal shape and energy. They can form four σ-bonds with other atoms or be filled with lone pairs of electrons.

The axes of sp3-hybrid orbitals are directed towards the vertices of a regular tetrahedron. The tetrahedral angle between them is 109 ° 28 ", which corresponds to the lowest repulsive energy of electrons. Also, sp3-orbitals can form four σ-bonds with other atoms or be filled with lone pairs of electrons.

Hybridization and molecular geometry

The concept of hybridization of atomic orbitals underlies the theory of repulsion of electron pairs Gillespie-Nyholm. Each type of hybridization corresponds to a strictly defined spatial orientation of the hybrid orbitals of the central atom, which allows it to be used as the basis for stereochemical concepts in inorganic chemistry.

The table shows examples of the correspondence between the most common types of hybridization and the geometric structure of molecules on the assumption that all hybrid orbitals are involved in the formation of chemical bonds (there are no lone electron pairs).

Hybridization type	Number hybrid orbitals	Geometry	Structure	Examples of
sp	2	Linear		BeF 2, CO 2, NO 2 +
sp 2	3	Triangular		BF 3, NO 3 -, CO 3 2-
sp 3	4	Tetrahedral		CH 4, ClO 4 -, SO 4 2-, NH 4 +
dsp 2	4	Squared		Ni (CO) 4, XeF 4
sp 3 d	5	Hexahedral		PCl 5, AsF 5
sp 3 d 2	6	Octahedral		SF 6, Fe (CN) 6 3-, CoF 6 3-

Links

Literature

• Pauling L. The nature of the chemical bond / Per. from English M. E. Dyatkina. Ed. prof. Ya.K. Syrkin. - M .; L .: Goskhimizdat, 1947 .-- 440 p.
• Pauling L. general chemistry... Per. from English - M .: Mir, 1974 .-- 846 p.
• Minkin V.I., Simkin B. Ya., Minyaev R.M. The theory of the structure of molecules. - Rostov-on-Don: Phoenix, 1997 .-- S. 397-406. - ISBN 5-222-00106-7
• Gillespie R. Geometry of molecules / Per. from English E. Z. Zasorin and V. S. Mastryukov, ed. Yu.A. Pentina. - M .: Mir, 1975 .-- 278 p.

see also

Notes (edit)

Wikimedia Foundation. 2010.

By the nature of the overlap, sigma σ-and pi-bonds - π are distinguished. σ-bond- it is a bond in which the overlap of atomic orbitals occurs along the axis connecting the atomic nuclei... The sigma bond can be formed by all types of orbitals. Between two atoms in a chemical particle is possible only one σ-bond... Overlapping parallel atomic orbitals perpendicular to the bond axisπ-bonds are formed. Pi-Link: Complementary to Sigma Link. A single link is always a sigma link. Double bond - consists of 1 sigma and 1 pi bond. Triple bond: 1 sigma and 2 pi bonds.

Single (σ)	Double (σ + π)	Triple (σ + π + π)
С – С С – Н С – О H – Cl	C = O C = C O = O	С≡С С≡N N≡N

Hybridization

If an atom is bonded to other atoms by EQUAL BONDS, but orbitals are involved in their formation different types, then the HYBRIDIZATION method is used.

Example:The CH 4 molecule has the shape of a regular tetrahedron, in which all 4 bonds have the same length, strength, and are at the same angles to each other.

However, in a tetravalent carbon atom, electrons are located in three p-orbitals and one s-orbital. They are different in energy, shape and are located differently in space.

For explanation, the concept of HYBRIDIZATION is used:

From four atomic orbitals, 4 new ones are formed,

hybrid orbitals, which in space are located AT THE MAXIMUM DISTANCE OF EACH OTHER. It is a regular tetrahedron, the angles between bonds are 109 ° 29´.

Since one s and three p-shells participate in the formation of four bonds, this type of hybridization is denoted sp 3

Depending on the number and type of orbitals that take part in hybridization, the following types of hybridization are distinguished:

1) sp-hybridization. One s-orbital and one p-orbital are involved. The molecule has a linear structure, the bond angle is 180 0.

2) sp 2 -hybridization. One s-orbital and two p-orbitals are involved. The molecule is located in a plane (the ends of the hybrid orbitals are directed to the vertices of an equilateral triangle), the bond angle is 120 0.

3) sp 3 -hybridization. One s-orbital and three p-orbitals are involved. The molecule has a tetrahedral shape, the bond angle is 109.28 0.

How to determine the type of hybridization?

1. Hybridization involves sigma bonds and INDIVIDUAL IONIC PAIRS.

2. The total number of participating sigma bond orbitals + electron pairs = the number of hybrid orbitals and determines the type of hybridization.

Exercise: determine the type of hybridization of the carbon atom in the phosgene molecule.

O = C - Cl

1) carbon forms 2 single bonds (these are sigma bonds) and one double bond (sigma + pi). All 4 electrons of carbon participate in the formation of these bonds.

2) thus, THREE SIGMA bonds will take part in hybridization. it sp 2 - hybridization, the molecule has the form flat triangle. The pi-link is located perpendicular to the plane of this triangle.

One of the tasks of chemistry is the study of the structure of matter, including the elucidation of the mechanism of formation different connections from simple substances formed by atoms of one chemical element... Features of the interaction of atoms, more precisely, their oppositely charged components - electron shells and nuclei - are described as Various types chemical bond. So, substances are formed through a covalent bond, for the description of which in 1931 the American chemist L. Pauling proposed a model of hybridization of atomic orbitals.

The concept of a covalent bond

In those cases when in the process of interaction the formation of a pair of valence electron clouds common for two atoms occurs, they speak of a covalent bond. As a result of its occurrence, the smallest particle of a simple or complex substance is formed - a molecule.

One of the features of a covalent bond is its directionality - a consequence complex shape electron orbitals p, d and f, which, not having spherical symmetry, have a certain spatial orientation. Another one important feature of this type of chemical bond - saturation, due to a limited number of external - valence - clouds in the atom. That is why the existence of a molecule, for example, H 2 O, is possible, but H 5 O is not.

Types of covalent bonds

The formation of common electron pairs can occur different ways... In the mechanism of covalent bond formation, an important role is played by the nature of cloud overlap and the spatial symmetry of the resulting cloud. According to this criterion, L. Pauling proposed to distinguish the following types:

• The sigma bond (σ) has the highest degree of overlap along the axis passing through atomic nuclei... Here the cloud density will be at its maximum.
• Pi-bond (π) is formed with lateral overlap, and the electron cloud, accordingly, has the highest density outside the axis connecting the nuclei.

These spatial characteristics are of great importance insofar as they correlate with the energy parameters of the covalent bond.

Features of polyatomic molecules

The concept of hybridization was introduced by Pauling to explain one of the features of covalent bonds in polyatomic molecules. It is known that the bonds formed by the central atom in such molecules are the same in spatial and energy characteristics. This happens regardless of which orbitals (s, p or d) are involved in the formation of a common electron pair.

Very convenient and good example the carbon atom serves to illustrate this phenomenon. When entering into a chemical bond, an atom in an excited state has 4 valence orbitals: 2s, 2p x, 2p y and 2p z. The last three differ from the 2s orbital in energy and shape. Nevertheless, in a molecule, for example, methane CH 4, all four bonds are completely equivalent and have bond angles of 109.5 ° (while the p-orbitals are located at 90 ° angles). In other carbon compounds, bond angles of 120 ° and 180 ° are found; in molecules containing nitrogen (ammonia NH 3) and oxygen (water H 2 O) these angles are 107.5 ° and 104.5 °. The appearance of such bond angles also required an explanation.

The essence of the phenomenon

The idea of ​​hybridization is the formation of averaged orbitals by overlapping electron clouds of different types with close energies - s, p, sometimes d. The number of resulting - hybrid - orbitals corresponds to the number of overlapping clouds. Since the orbital is the determining probability of finding an electron at one point or another of an atom, a hybrid orbital is a superposition of wave functions that occurs as a result of electronic transitions when an atom is excited. It leads to the appearance of equivalent wave functions differing only in directionality.

Hybrid orbitals are equivalent in energy and have the same shape in the form of a three-dimensional figure, which has a strong asymmetry relative to the nucleus. Less energy is spent on hybridization than is released during the formation of a strong covalent bond with hybrid orbitals; therefore, such a process is energetically favorable, that is, the most probable.

orbital hybridization and molecular geometry

Possible different options overlapping (mixing) of the outer electron clouds in the atom. The most common types of orbital overlays are:

• Sp 3 -hybridization. This option is realized when one s- and three p-orbitals are superimposed. It results in four hybrid orbitals, the axes of which are directed for any pair at angles of 109.5 °, corresponding to the minimum mutual repulsion of electrons. When these orbitals enter into σ-bonds with other atoms, a tetrahedral molecule is formed, for example, methane, ethane C 2 H 6 (a combination of two tetrahedra), ammonia, water. In the ammonia molecule, there is one, and in the water molecule, two of the vertices of the tetrahedron are occupied by lone electron pairs, which leads to a decrease in the bond angle.
• Sp 2 hybridization occurs when one s and two p orbitals are combined. In this case, the three hybrid orbitals are located at 120 ° angles in the same plane. A similar triangular shape has, for example, boron trichloride BCl 3 molecules, which is used in different technologies... Another example - an ethylene molecule - is formed due to an additional π-bond between carbon atoms, in which one p-orbital is non-hybrid and oriented perpendicular to the plane formed by two triangles.
• Sp hybridization occurs when one s and one p orbital is mixed. The two hybrid clouds are at 180 °, and the molecule has a linear configuration. Examples are molecules of beryllium chloride BeCl 2 or acetylene C 2 H 2 (in the latter, two nonhybrid p-orbitals of carbon form additional π-bonds).

There are also more complex variants of hybridization of atomic orbitals: sp 3 d, sp 3 d 2 and others.

Role of the hybridization model

Pauling's concept gives a good qualitative description of the molecular structure. It is convenient and clear, successfully explains some of the features of covalent compounds, such as the value of bond angles or equalization of the length of a chemical bond. However, the quantitative aspect of the model cannot be considered satisfactory, since it does not allow making many important predictions regarding physical effects associated with the structural features of molecules, for example, molecular photoelectron spectra. The author of the concept of hybridization himself noted its shortcomings in the early 1950s.

Nevertheless, the model of hybridization of atomic orbitals played an important role in the formation of modern concepts of the structure of matter. On the basis of it, more adequate concepts were developed, for example, the theory of repulsion of electron pairs. Therefore, of course, the hybridization model was an important milestone in the development of theoretical chemistry, and in the description of some aspects of the electronic structure of molecules, it is quite applicable at the present time.